1. Field of the Invention
The present invention is related to a novel lubricating oil for compression-type refrigerators, a novel lubricating oil composition containing the oil and refrigerant, and a method for effecting lubrication of a refrigerator. More particularly, the present invention relates to a lubricating oil for compression-type refrigerators which comprises as the main component thereof a polyvinyl ether compound having excellent compatibility with hydrogen-containing Flon compounds ["a Flon compound" means a chlorofluorocarbon (CFC), a hydrofluorocarbon (HFC) and a hydrochlorofluorocarbon (HCFC) in general], such as 1,1,1,2-tetrafluoroethane, difluoromethane, pentafluoroethane (referred to as Flon 134a, Flon 32 and Flon 125, respectively, hereinafter) and the like, which can be used as the refrigerant to replace compounds causing environmental pollution, such as dichlorofluoroethane (referred to as Flon 12, hereinafter) and the like, and having an excellent lubricating property.
2. Description of the Related Arts
Compression-type refrigerators are generally constituted with a compressor, a condenser, and expansion valve and an evaporator and has a structure that mixed fluid of refrigerant and lubricating oil is circulated in this closed system. In the compression-type refrigerator generally temperature is high in the compressor and low in the refrigerator although the actual condition is different depending on kind of apparatus and it is generally required that the refrigerant and the lubricating oil are circulated in the system without causing phase separation in the wide range of temperature. When the phase separation occurs during the operation of the refrigerator, life and efficiency of the apparatus are adversely affected to a great extent. For example, when the phase separation of the refrigerant and the lubricating oil occurs in the part of the compressor, lubrication of the moving parts is deteriorated and seizure occurs to cause decrease of life of the apparatus to a great extent. When the phase separation occurs in the evaporator, efficiency of heat exchange is decreased because of the presence of lubricating oil of high viscosity.
Because the lubricating oil for refrigerators is used for the purpose of lubricating moving parts in refrigerators, the lubricating property is naturally important. Particularly, because the temperature in the compressor is high, the viscosity which can hold the oil film necessary for the lubrication is important. The required viscosity is different depending on the kind of the compressor used and conditions of use and it is generally preferable that viscosity (kinematic viscosity) of the lubricating oil before mixing with the refrigerant is 5 to 1000 cSt at 40.degree. C. When the viscosity is lower than this range, oil film becomes thin to cause insufficient lubrication, and, when the viscosity is higher than this range, efficiency of the heat exchange is decreased.
Electric refrigerators have the motor and the compressor built into a single body and lubricating oil for them is required to have a high degree of electric insulating property. In general, volume specific resistance of 10.sup.12 .OMEGA..multidot.cm or more at 80.degree. C. is required. When the resistance is lower than this value, possibility of leak of electricity arises.
Furthermore, high degree of stability is required for lubricating oil. For example, when organic acids are formed by hydrolysis, corrosion and abrasion of apparatus tend to occur although degree of such occurrences depends on the amounts of the acids.
As the refrigerant for compressor-type refrigerators, mainly Flon 12 has heretofore been used and, as the lubricating oil, various kinds of mineral oil and synthetic oil satisfying the required properties described above have been used. However, Flon 12 is being more rigorously restricted world-wide because it brings environmental pollution such as the rupture of the ozone layer. By this reason, hydrogen-containing Flon compounds represented by Flon 134, Flon 32 and Flon 125 are attracting attention as the novel kinds of the refrigerant. The hydrogen-containing fluorocarbons, particularly Flon 134a, are preferred as the refrigerant for compression-type refrigerators because they have little possibility of causing the rupture of the ozone layer and can replace Flon 12 with little change of the structure of refrigerators which have heretofore been used.
When a hydrogen-containing Flon compound described above, such as Flon 134a and the like, is adopted as the refrigerant for compression-type refrigerators to replace Flon 12, a lubricating oil having good compatibility with the hydrogen-containing Flon compound, such as Flon 134a, Flon 32, Flon 125 and the like, and good lubricating property satisfying the requirements described above is naturally required. However, because the lubricating oils used in combination with Flon 12 heretofore do not have good compatibility with the hydrogen-containing Flon, such as Flon 134a, Flon 32, Flon 125 and the like, a new lubricating oil suited for these compounds is required. When a new lubricating oil is adopted in accordance with replacement of Flon 12, it is desired that major change of the structure of the apparatus is not necessary. It is not desirable that the structure of the currently used apparatus must have major changes because of a lubricating oil.
As the lubricating oil having the compatibility with Flon 134a, for example, lubricating oils of polyoxyalkylene glycols have been known. For example, Research Disclosure No. 17463 (October, 1978), the specification of the U.S. Pat. No. 4,755,316, Japanese Patent Application Laid Open No. 1989-256594, Japanese Patent Application Laid Open No. 1989-259093, Japanese Patent Application Laid Open No. 1989-259094, Japanese Patent Application Laid Open No. 1989-271491, Japanese Patent Application Laid Open No. 1990-43290, Japanese Patent Application Laid Open No. 1990-84491, Japanese Patent Applications Laid Open No. 1990-132176 to 132178, Japanese Patent Application Laid Open No. 1990-132179, Japanese Patent Application Laid Open No. 1990-173195, Japanese Patent Applications Laid Open No. 1990-180986 to 180987, Japanese Patent Applications Laid Open No. 1990-182780 to 182781, Japanese Patent Application Laid Open No. 1990-242888, Japanese Patent Application Laid Open No. 1990-258895, Japanese Patent Application Laid Open No. 1990-269195, Japanese Patent Application Laid Open No. 1990-272097, Japanese Patent Application Laid Open No. 1990-305893, Japanese Patent Application Laid Open No. 1991-28296, Japanese Patent Application Laid Open No. 1991-33193, Japanese Patent Applications Laid Open No. 1991-103496 to 103497, Japanese Patent Application Laid Open No. 1991-50297, Japanese Patent Application Laid Open No. 1991-52995, Japanese Patent Applications Laid Open No. 1991-70794 to 70795, Japanese Patent Application Laid Open No. 1991-79696, Japanese Patent Application Laid Open No. 1991-106992, Japanese Patent Application Laid Open No. 1991-109492, Japanese Patent Application Laid Open No. 1991-121195, Japanese Patent Application Laid Open No. 1991-205492, Japanese Patent Application Laid Open No. 1991-231992, Japanese Patent Application Laid Open No. 1991-231994, Japanese Patent Application Laid Open No. 1992-15295, Japanese Patent Application Laid Open No. 1992-39394 and Japanese Patent Applications Laid Open No. 1992-41591 to 41592 disclosed such lubricating oils. However, the lubricating oils of polyoxyalkylene glycols generally have low volume specific resistances and no example satisfying the value of 10.sup.12 .OMEGA..multidot.cm or more at 80.degree. C. has been disclosed yet.
As the compound having the compatibility with Flon 134a in addition to the lubricating oils of polyoxyalkylene glycols, lubricating oils of esters were disclosed in British Patent Laid Open No. 2216541, WO No. 6979 (1990), Japanese Patent Applications Laid Open No. 1990-276894, Japanese Patent Applications Laid Open No. 1991-128992, Japanese Patent Applications Laid Open No. 1991-88892, Japanese Patent Applications Laid Open No. 1991-179091, Japanese Patent Applications Laid Open No. 1991-252497, Japanese Patent Applications Laid Open No. 1991-275799, Japanese Patent Applications Laid Open No. 1992-4294, Japanese Patent Applications Laid Open No. 1992-20597 and the specification of the U.S. Pat. No. 5,021,179. However, it is inevitable that lubricating oils of esters form carboxylic acids by hydrolysis and this phenomenon causes corrosion of apparatuses. For example, rubber hoses are used in air conditioners of automobiles and lubricating oils of esters cannot be used because moisture penetrates through the rubber hoses. In electric refrigerators, although there is no possibility of penetration of moisture while they are used, lubricating oils are used for a long time without being changed to new oils and almost all amount of the moisture mixed at the time of production is used for hydrolysis, causing the problem. Because of these problems, a large extent of modification of the currently used apparatus or the apparatus for production is required when lubricating oils of esters for compression-type refrigerators is adopted and this situation is not desirable. As an oil for refrigerators having good resistance to hydrolysis, an oil composition for refrigerators characterized by containing an epoxy compound was disclosed in Japanese Patent Application Laid Open No. 1991-275799. However, the resistance to hydrolysis of this oil composition for refrigerators derives from the reaction of the epoxy group with water to form alcohol. When a large amount of water is contained, there is the possibility that the composition of the oil composition for refrigerators is changed to a large extent. Even when the content of water is small, the alcohol formed causes ester exchange reaction and there is the possibility that the composition of the oil composition for refrigerators is change to a large extent. Thus, this oil composition for refrigerators is not preferable.
Lubricating oils of carbonates were disclosed in Japanese Patent Application Laid Open No. 1991-149295, European Patent No. 421298, Japanese Patent Application Laid Open No. 1991-217495, Japanese Patent Application Laid Open No. 1991-247695, Japanese Patent Application Laid Open No. 1992-18490 and Japanese Patent Application Laid Open No. 1992-63893. However, the lubricating oils of carbonates have the same problem of hydrolysis as the lubricating oils of esters.
Thus, it is the real situation at present that a lubricating oil for the compression-type refrigerators having excellent compatibility with Flon 134a, excellent stability and lubricating property and a volume specific resistance at 80.degree. C. of 10.sup.12 .OMEGA..multidot.cm or more has not been discovered yet. Development of such a lubricant is strongly desired.